Colloidal semiconductor nanocrystals (NCs) have attracted great interest due to the ability to tailor their absorption and photoluminescence (PL) over a wide spectral range, by changing their size, shape and composition. In particular, II-VI and III-V semiconductors NCs are of importance due to their fluorescence, covering the visible to the near infrared (NIR) spectrum, which is appealing for a variety technological applications [1,2].
The optical behavior of the particles can be further modified by controlling their shape. For example, unlike spherical NCs, nanorods have been found to have linearly polarized emission. In addition, the rod shape of the particles enables electric field induced switching of the fluorescence [3,4]. These properties make semiconductor nanorods (NRs) highly desirable [3,5,6].
Furthermore, NRs display unique characteristics including low lasing thresholds associated with increased Auger lifetimes [7,8], large absorbance cross-sections, and linearly polarized absorption and emission [9,10]. These properties show promise for using NRs in applications such as lasing [7,11,12], bio-labeling [13,14], and polarized single-photon sources [15].
While II-VI semiconductor nanorods are grown via a surfactant control growth approach, this approach is difficult to realize for cubic structured semiconductor NCs such as the III-V semiconductors. In these NCs, chemically dissimilar surfaces are not obviously present due to the high symmetry of lattice, and as a result preferential binding of ligands, which is essential for the surfactant controlled growth mechanism, cannot be obtained. Previous works showed that III-V semiconductor rods can be grown via a solution-liquid-solid (SLS) mechanism, by using small metal NCs as catalyst for rod growth [16,17]. However, the presence of the metal particles strongly quenches the photoluminescence.
Recently, a new type of core-shell nanoparticles, known as seeded rods (SRs) was introduced [18-20], where spherical nanoparticle of one material is embedded within a rod of another material. Several SR systems were reported including CdSe cores embedded in CdS rods, forming type-I and quasi-type-II systems [20-24], and ZnSe cores embedded in CdS rods forming a type-II system [18]. These particles exhibit several advantageous properties typical for 1d systems, including linearly polarized emission [23,25], suppression of Auger nonradiative recombination [26], and large absorbance cross-sections [18,20,23].
The synthesis of such structures is performed by two consecutive steps, where NCs seeds are first synthesized, and then the seeds are rapidly injected into a hot solution of precursors and ligands for the formation of a rod shell around them. Such dot-rod heterostructures are highly crystalline and uniform and exhibit strong and stable PL emission. However, in order to achieve good optical properties, the core and shell materials should have a low lattice-mismatch and generally also similar crystal type, which limits the variety of structures that can be constructed in this manner. Core/multishell NPs were introduced for spherical shaped particles, but the utilization of this concept in rod shaped systems and in particular in seeded rod was never previously performed.
Core\shell semiconductors nanoparticles are more stable for photoluminescence and have higher quantum efficiency due to the shell passivation of the dangling bonds. Bawendi et al. [27] presented synthesis of CdSe quantum dots coated with a ZnS shell and having photoluminescence quantum yield of up to 50% and a narrow size distribution. These CdSe dots were synthesized through a typical TOP\TOPO synthesis. A size selective precipitation of these dots was performed by means of centrifugation. A ZnS shell was deposited by a drop-wise addition of the Zn and S precursor mixture. It was further contemplated that the process may be applicable also for CdTe and CdS cores and for ZnSe shell. Banin et al. [16] discloses different III-V II-VI core shell combinations.
Treadway et al. [28] expanded the method for various kinds of semiconductors materials including II-VI III-V families. In this method all the layer precursors are added simultaneously as described therein. In 2003, Peng et al. [29] demonstrated a method for preparing core/shell structures. In the method the layers are added successively and each layer is added in two steps, one for the anion and second for the cation.
In 2005, Banin et al. [30], using a layer by layer method, showed a new kind of material, composited of a core coated with a multilayered shell. This work presented III-V\II-VI\II-VI spherical core\shell1\shell2 materials, having high quantum efficiency and high photostabilty improved over the previous core/shell. In addition, this method enables a variety of combinations between different materials and crystal structures. Peng et al. [29]described syntheses of II-VI\II-VI, III-V\II-VI, II-VI\III-V and III-VI\III-V spherical core\shell nanocrystals, creating quantum dots and quantum shells (reverse type-I). The same method is used for synthesizing of core\multishell structures, creating quantum wells and dual emitting quantum dots. Furthermore, core\shell and core\multishell nanocrystals doped with transition metals (Mn, Fe, Cu, etc.) are synthesized using the same method.
Rod-shaped semiconductors nanoparticles have unique properties due to their ID confinement. These materials hold high polarization which can be used in different applications.
Alivisatos et al. [31] describes a process for the formation of II-VI semiconductor nanorods, in which shape control is achieved by adding different surfactants. The balance between the surfactant induces different shapes. Group II metal (Zn, Cd, Hg) and group VI element (S, Se, Te) are added, together or separately, to the heated mixture of surfactants, followed by decreasing the temperature to allow crystal growth.
Alivisatos et al. [32] suggested the same process for the formation of III-V semiconductor nanorods.
Rod/shell materials have better optical properties. However, this presents a significant synthetic challenge, as the rod shape must be preserved even though it is not thermodynamically stable. Several examples for core/shell NRs were synthesized, showing improved quantum efficiencies of around 30% [33,34]. These materials were reported by Alivisatos et al. [35] describing methods for synthesizing CdSe rod shaped core coated with a graded CdS\ZnS shell, and by Banin et al [34] growing ZnS shells. The shell precursors are added drop-wise. In this case the resulting structure has a thin shell layer and coats the rod essentially with even thickness on all sides. The invention contemplates the same structure for rod shaped cores and conformal shells of different semiconductor groups (II-VI, III-V, IV).